The applications of such reactive polymers, upon which chemical or biochemical products or cells can be fixed according to the case, are numerous, but until now these beads of copolymers (crosslinked polymer or gels) could only be prepared in two steps. Indeed, in the known conditions of free radical polymerisation in suspension, this is performed either with monomers--like styrene in organic solution and in emulsion in water, or with more polar monomers--like the acrylamides--in aqueous solution and in emulsion in mineral oil, and at temperatures ranging from 30.degree. to 80.degree. C. for reaction times of 4 to 30 hours. In such conditions almost no functional group sufficiently reactive to be capable of coupling subsequently, according to the case, a chemical or biochemical product or a cell, is stable enough and it was not possible to introduce into the polymerisation medium monomers bearing the desired group without the, at least partial, destruction of this group by hydrolysis. Also the preparation was recommended in the first step of a polymer or copolymer bearing lateral groups stable in aqueous medium but capable of subsequently reacting with divalent entities which, upon fixation, create the desired reactive sites, and that in a second step called "activation" the reactive groups are introduced; this step frequently even being carried out by the user alone, with all the difficulties which that involves. It is possible to cite as examples the chloromethylation of polystyrene gels, the formylation of acrylamide copolymers used for coupling chiral amines and providing resins suitable for filling high pressure liquid chromatography columns used for the separation of enantiomers (J. Liquid Chromatog. 1 67 p.761 (1978)), or further the activation of acrylamide copolymers by cyanogen bromide to obtain gels that bind biological macromolecules. (FR-A-2.398.762).